Metal powder



M Me ""1 1 2,001,134

"UNITED STATES PATENT OFFICE j 2,001,134 v I rm'rar. rownEa Charles Hardy, Pelham Manor, N. Y., aelixnor to Hardy Metallurgical Company, a corporation of Delaware I No'Drawln g. Application February 6, 1938,

Serial No. 655.479

4 Claims. (91. ':5-1)

This invention relates to metal powders, and to various mechanical working operations to has for its object the provision of improvements place the metal into useful fabricated shapes. To in the art'of manufacturing useful objects with this ,end; the metal may be forged, swaged, metal powders. rolled, drawn and the like, hot or cold. In some 5 In the manufacture of many useful metal obinstances, the metal is initially brittle and makes 5 jects, it is customary to melt one or more metals such mechanical working practically impossible. and cast the same in appropriate dies or molds. This is particularly true of alloys of certain kinds, While this'practice has many advantages, it likesuch as an alloy compounded of high melting wise offers many difliculties that metal workers and low meltingtemperature metals.

1; would like to avoid. The preparation of a molten Many of the difliculties enumerated, and othmetal, or alloy. of metals, isaccompanied by ers, may be substantially avoided in the practice certain dangers that may seriously impair the of the present invention; which involves the use ultimate metal'object tobe made therefrom. At of metal powders initially, instead of molten the temperatures required to melt the metal or metal.

,5 metals, the molten metal is apt to pick up im- The present invention contemplates a methpurltles of various kinds. The fuel employed to 0d of manufacturing a metal object according to melt the metal may add sulfur and other 1111- which a portion of the me lin the form of finepurities. The molten metal may pick up i m- 1y divided powder with m tallic surfacesis placed purities from its container. Unless extreme preunder suitable pressure to cause the powder to cautions are taken, the molten metal will become cohere into a body of relatively low density. This 20 .oxidized on contact with atmospheric air. Durbody of coherent powder is then subjected to a ing the melting operation, some of the metal may heat-treatment operation until the particles of vaporize or fume'and thus be lost. The proporpowder are sintered or fused into a substantially tions ofmetals present are then unbalanced. It homogeneous mass. The heat-treated mass is is particularly difficult to compound an alloy in then mechanically worked under appropriate 2 this manner when a metal of high melting point pressure to produce an object of desired high is to be joined with a volatile metal or one of density; which may be followed or accompanied low melting point. i with a further heat treatment operation, and

Casting difliculties are encountered when the perhaps a further mechanical working step.

molten metal or metals is poured into a die or Metal powders of various kinds of manufacture 30 mold. As the molten metal freezes, segregations may be employed in the practice of the invention. of various kinds may form throughout the body At the present time the following metal powders of metal, which may ultimately lead to the disare being produced commercially: iron, chrointegration of the object; or at least seriously mium, nickel, copper, silver, tin, zinc, molybdenum,

impair its physical -'and chemical characteristics. tungsten, etc. The more conventional methods Provision must be made for shrinkage that takes of powder making involve the crushing and grindplace when the molten metal solidifies into afixed ing of metal ingots or the like to substantially shape. Needless to say, further impurities may minute or microscopic powder particles. This be picked up by the molten metal during the product is generally of a flake-like consistency.

40 casting operation. This is particularly true when Metal powders are also made by reducing the 40 die-cast articles 'are made. At the high tempurified oxide of the metals in a correct atmosperatures required, the molten'metal gradually phere ordinarily hydrogen, hydrogen and nitroeats the metal die employed. This therefore gen. The deposition ofpulverulent metal directly serves not only .to wear away the die, but toadd an electrolytic tank offers an excellent source impurities to the finished article. of powder. The temperature and concentration 45 After the metal, or metals, has been cast, furof the electrolyte, the current density and voltther difficulties are frequently met in subsequent age, the nature and disposition of anodes and finishing operations. Many metal castings must cathodes must all be kept under control and withbe subjected to intricate and costly machining in proper limitations in order to produce a dense 60 operations of various kinds. Inconveniences are deposit at a, uniform rate. According to one met whether or not the casting lends itself readmethod of electrodeposition; a catalyst is introily to machining; but such difiiculties are, of duced into the electrolyte, and the amount of course, worse if the casting has physical characcopper in suspension and the temperature of the teristics that resist machining to a, great extent. electrolyte are closely controlled. According to Many metals and alloys of metals are subjected another procedure, the copper powder is de- 55 posited with a very high current density. The powder is washed, dried and rubbed after which it may be carefully separated into sizes by a series of screens and bolting cloths of closer and closer weave. The very fine particles may be removed and classified by an ascending air current. In this manner, a closely graduated powder of preferable particle size may be obtained. Furthermore, the metal powder has substantially clean metallic surfaces.

The metal powders are readyfor compression, either as an individual metal or as a controlled mixture of two or more metals. The initial compression may be effected at a pressure adequate to form the powder into a coherent body of desired density for the ultimate article; or initial compression may be sufficient only to cause the powder to cohere so that it may be in condition for subsequent treatment, such as heat treatment; to be followed with further compression or other working under'still greater pressure.

In a present preferred practice of the invention the metal powder, or mixture of powders of various metals, is subjected to an extrusion operation. To this end, the sized powder is mixed by a tumbler or other standard mixing device. To increase the fluidity of the powder, so that it may be more readily forced through a die, an appropriate vehicle is added to increaseits flow characteristics. This vehicle may be regarded as a lubricant, which preferably also acts as a binder. I have found that metal soaps are peculiarly adapted for this purpose, such as the stearates and oleates of zinc, copper and the like. Paraffins, resin and camphor, etc. may also be employed. The lubricating and binding agent is thoroughly incorporated with the metal powder to obtain optimum fluidity. When powders of two or more metals of considerable difierence in specific gravity are mixed, they tend to be selectively grouped; the powder of one metal being bunched while that of another is likewise bunched. Segregation of this kind is objectionable, and makes for non-uniformity of the ultimate product. By using a binder of the kind contemplated, sufficient adhesion is provided between contacting particles, so that segregation cannot take place-and the fiow characteristics of the mixture are improved. The metal powder thus treated is extruded through an appropriate die, which causes the extruded powder to cohere at relatively low density. The invention also contemplates that metal powder so treated may be initially compressed into an article of the ultimate required density. Articles corresponding to diecasting may be made in this manner.

As a result of the compression, extrusion or other pressure operation, the metal powder is placed into a coherent mass. The coherent mass is then subjected to a suitable heat-treatment operation to cause the individual particles to fuse or sinter together to form a substantially homogeneous product. Any suitable heat-treatment procedure may be employed. Thus, the coherent mass may be heated in a non-oxidizing atmosphere. For this purpose, the coherent mass may be surrounded by inert or reducing gases. The

desired heat-treatment operation may also be' conducted by passing a suitable current of electricity through the coherent mass. Due to the microscopic voids between the powder particles, resistance is offered to the passage of the electrical current, and the temperature of the mass is elevated. The heat-treatment of the coherent mass may also be conductecl'in fused salts, such contact with metal. This welding action may be" done so effectively that subsequent fractures in test or overload will cross through the middle of the original grains rather than atthe welded contacts. The temperature employed may usually be fixed at or below approximately two-thirds of the indicated temperature required to melt a given mixture of two or more different metals. When the powders of two or more metals are compounded in this manner, the composite product may be regarded as a synthetic alloy.

As pointed out above, the heat-treated mass of metal may or may not be subjected to a further working operation, depending upon the ultimate metal product to be produced. There is no limitation on the number of repressings, heat-treatments, and re-workings that may be employed. It is frequently desired to increase the density of the heat-treated object. The powders may be initially compressed into a coherent body of relatively low density. After the heat-treatment operation, the low density object is advantageously mechanically worked at increasing pressures in order to yield an ultimate product of higher density. This procedure tends toward the economy of considerable energy as well as in the saving of wear and tear on tools over the usualpractice when dealing with molten alloys. Iron powder, for example, may be extruded to form a coherent mass having a density of 6.1. After heat-treatment and subsequent working at higher pressures, the density may be elevated to 6.95. This latter figure, it will be observed, closely reaches the normal density 7 of iron when cast. In a similar manner, copper has a density of about 8.4 when cast. Extruded copper powder'may be made to cohere at a density of about 4.2 with 2 tons pressure, which, after heattreatment and subsequent workings at 250 tons, may be elevated to 8.3. By appropriate repressmgs and heat-treatment, such heat-treated cop; per has been made to have a density of 8.5 to 8.6, which is higher than cast copper.

Comparable results are obtained in conductivity tests made on copper powders that have been fabricated into test specimens. The extruded coherent material may be made to have a'conductivity of 30.50% of a standard cast specimen;

Cast metals, moreover, as pointed out above,

may'have certain disadvantages, particularly in alloy form. A'high zinc-copper brass alloy cannot be drawn readily because it is too brittle. A corresponding zinc-copper composition made from powders in accordance with the present invention may, however, be readily drawn. The problem of brittleness is avoided in this manner. Substantially the same situation obtains with respect to cast alloys high in chromium, cobalt, tungsten, molybdenum, and similar metals that add hardness to alloys. I

The practice of the invention may be considered with reference to a few examples:

An iron-chromium-nickel composition was prepared consisting of 8% nickel, 18% chromium and the balance iron powder. The three metal powders were thoroughly mixed and compressed under a,pressure of 50 tons into a bar treated bar was then cold rolled to a thinness of 0.03 inch.

A copper-tin composition was prepared consisting of 90 parts copper, 10 parts tin, and 1 to 2 parts copper stearate, all in powder form. The powders were thoroughly mixed and extruded to form a inch rod. This rod was heattreated at 750 C. in city gas for 4 hours. The heat-treated rod was then hot-drawn with subsequent annealing to a 0.004 inch bronze wire.

A steel article of specified weight and dimensions was made as follows: An amount of iron powder containing 0.8% carbon conforming in weight to that of the ultimate article desired was compressed into a briquette of suitable configuration under 30 tons pressure. The coherent mass of material was then heat-treated in a hydrogen furnace for 2 hours at 1800 F. The tensile strength of the article at this stage was 36,000 pounds per square inch. The heat-treated briquette was then placed in a split die and cold forged to a finished article of the configuration desired. It then had a tensile strength of 63,000 pounds per square inch andwas ready for use without any machining.

From the foregoing it will be seen that compressed nietal objects of the kind contemplated offer numerous advantages. The porosity of the final mass is under control. Stainless alloys, for example, may be produced that have half the specific gravity of rolled steel. The grain size may be placed under accurate control. Metal products of the highest purity may be produced. Thus, iron alloys substantially free of sulfur, phosphorous, carbon and nitrides may be made. Alloys of constant composition may be produced. This is very important for electrical alloys. Thus, a 90-10 bronze and a -30 Monel metal may be made of powdered metal to a uniformity entirely unapprcachable by foundry methods. Contact points made of silver and molybdenum, and of silver and nickel, are otherexamples. Alloys and mixtures of metals immiscible in the liquid state or having great differences in melting points may be produced. Instances of this kind are lead and copper, copper and tungsten, copper and chromium, copper and molybdenum, etc. Mixtures of metal and non-metal may be effected. For example, tungsten carbide and cobalt; copper and graphite, copper and porcelain, copper and alundum mixtures, etc. The size of the completed article may be held to close limitations. The dies may be cut to any desired degree of accuracy and the presses refined in design. The finished article can be brought within exceedingly close limits by restriking or coining in a sizing die-limits in some cases as close as 0.0005 inch. Manufacturing losses are very small. Slagging losses incident to melting are avoided. There is a diminution in the amount of crop and oxide loss resulting from rolling; or the flash or trimming loss of forging and stamping; whereas when molten metals are employed, frequently only about one-half of the original metal appears in the finished part, all of the metal in the practice of the present invention may be placed in the finished article. Stainless steel strips may be rolled out of powder-billets, giving several important advantages. In the first place, the chromium and nickel content of the synthetic a1- loy may be held more closely than by conven tional furnace practice; secondly, the strips are more homogeneous; and, in the third place, powder-billets are easier to roll due to initial lower density of the billet. Useful bior poly-metal products may be manufactured. Composite tools of various kinds may be manufactured, for example, with a strong steel base and a layer of excessively hard alloy on top. A considerable saving in valuable raw material may be made. These and other advantages, too numerous to list, are possible in the practice of the invention.

I claim:

1. A method of manufacturing a metal object of die-cast configuration which comprises forcing a portion of the metal in the form of finely divided closely graded powder with metallic surfaces which are coated with metallic soap under adequate pressure into a die to cause the powder to cohere at an appropriate density into an object of the desired configuration, and then subjecting the object thus formed to a heat-treatment operation until substantially all of the metallic soap is decomposed and, the powder particles are welded into a substantially homogeneous mass.

2. In a method of manufacturing a metallic object from a metal powder the improvement comprising, mixing said metal powder with a me-' tallic soap the metal of which conforms to that of the powder prior to compressing the powder into a coherent body and heating the coherent body until substantially all of the metallic soap is decomposed.

3. A method of manufacturing a metallic object which comprises coating powders of the heavy metals with the soap of a heavy metal, extruding the coated powders of the heavy metals under pressure to form a coherent mass of relatively low density, sintering the coherent mass, until substantially all of the metallic soap is decomposed and subjecting the sintered mass to compression to increase its density.

4. A method of manufacturing a metallic object which comprises coating finely divided metallic particles with a soap of a heavy metal, extruding the coated metal particles under pressure to form a coherent mass, and decomposing substantially all of the metallic soap by sintering said mass at a relatively elevated temperature.

CHARLES HARDY. 

